1. Field of the Invention
The invention relates to the field of telecommunications hardware, and more specifically to an environmentally secure enclosure for protecting splices between fiber optic communications cables.
2. Description of the Related Art
Over the last decade, the use of fiber optic cable for the transmission of voice, video, and data services has become increasingly common. Fiber optic cables come in a variety of types, including optical ground wire (OPGW) and all-dielectric self-supporting (ADSS) cable. Optical ground wire is designed for aerial installation on electrical transmission line towers where it replaces the existing ground cables. All dielectric self supporting cable may also be used in aerial installations, and in addition may be installed underground, either in a duct or by direct burial. The all-dielectric construction of these cables provides resistence to lightning and static discharge, negating the need for grounding. Cable which is not all-dielectric requires the use of an isolated ground.
No matter which type of fiber optic cable is used, the cables must inevitably be spliced. Splicing is required during initial installation between cable runs and at points where the cable branches. After installation splices are often required to repair breaks in the cable. A single fiber optic cable can contain hundreds of individual fibers which must be separately spliced, either by fusion or by mechanical connectors. The splices between the fibers are susceptible to degradation by moisture, and therefore the splices must be protected from the elements. It is for this reason that fiber optic cable splices are provided with weatherproof enclosures.
U.S. Pat. No. 5,007,701 (""701 patent), issued Apr. 16, 1991 to Gene Roberts and commonly owned with the present application, discloses a splice closure apparatus in which a rigid, cylindrical and weather impervious outer structure is open at both ends. A pair of end caps each include an elastomeric sealing material sandwiched between a pair of rigid plates. At least one of the end caps has openings for the passage of cables therethrough into the interior of the closure where an anchoring member is positioned to receive the cable ends and secure a splice. The elastomeric sealing material in each end cap is compressed against the sides of the outer cylinder by the plates on either side when a number of through bolts are tightened, thus forming a weather tight seal between each end cap and the cylinder wall. In addition, in each end cap with cable openings, the elastomeric material is simultaneously urged tightly against the cables, thus also forming a weather tight seal between the end cap and the cables.
U.S. Pat. No. 5,479,554 (""554 patent), issued Dec. 26, 1995 to Gene Roberts and also commonly owned with the present application, discloses an improved version of the splice closure apparatus which is specially adapted for use with optical ground cable. As is the case with the closure apparatus of the ""701 patent, the closure of the ""554 patent includes a weather impervious outer cylinder made of an impact and corrosion resistant material which is open at both ends and designed to hold a splicing assembly including a pair of sealing end caps, one for each end of the cylinder. A first of the end caps is similar in structure to that of the ""701 patent, i.e. an elastomeric sealing material is sandwiched between two rigid plates bolted together with the bolts extending through the elastomeric material. When the end cap bolts are tightened, the elastomeric material is expanded outward to sealingly engage the outer cylinder. The splice assembly includes a skeletal frame connected between the first end cap and the second end cap through which cables to be spliced are introduced into the splicing assembly and closure. The second end cap also has an elastomeric material layer sandwiched between an inner and an outer plate, but each of the plates include four cable slots leading from the outside thereof to respective cable through holes in the elastomeric layer. The elastomeric layer is segmented such that a pair of outer sections between respective slot pairs are removable. The second end cap has attached thereto a pair of separable outer cable clamps sized to accommodate the outer, grounding sleeve of the optical ground cable and a pair of separable inner cable clamps, sized to accommodate the smaller, inner protective tube surrounding the optical fiber bundles. With this arrangement, with the outer elastomeric material segments removed, a continuous cable can be inserted into the cable clamps via the slots and thus into the splicing assembly without the need to sever the cable. The cable can then be securely clamped in place via both inner and outer cable clamps and the elastomeric material segments replaced prior to installation of the splicing assembly in the outer protective cylinder.
Attached to the skeletal frame are a pair of cable returns which encompass an arc segment with a diameter allows the fiber optic bundle to be doubled back on itself without exceeding a critical angle at which light will no longer be transmitted along the fiber strands. In addition, a centrally mounted splice tray support is provided for anchoring a variety of splicing trays for facilitating splicing operations.
The cylindrical enclosures of the ""701 and ""554 patents have worked very well over the years, but recently the demand for additional bandwidth has increased dramatically, requiring the use of cables with higher fiber counts. With increasing fiber counts, there has emerged a need for greater capacity within the splice enclosure, as each splice requires a discrete amount of physical space. In addition, there must be room enough within the splice enclosure to bend the cables without disrupting the light flow through the fibers. It has become apparent that a simple cylindrical enclosure does not provide the optimum internal dimensions to meet the needs of current fiber optic technology.
Previous attempts to address this problem have used enclosures having generally rectangular cross-sectional shapes. An example of this type of enclosure is the AFL OPTI-Guard splice enclosure produced by ALCOA. These rectangular enclosures provide additional splice capacity as compared to the cylindrical variety of splice enclosure, however the rectangular shape of these enclosures is not suitable for use with the proven expansion seal used in the ""701 and ""554 patents. This is true for at least two reasons: first, the flat sides of the rectangular enclosure are not sufficiently rigid to prevent distortion upon application of pressure from the seal; and second, the elastomeric sealing member cannot expand into the square corners of the enclosure sufficiently to guarantee a positive seal.
What is needed is a fiber optic splice enclosure which combines the effective expansion type seal of the ""701 and ""554 patents with a shape that provides the additional capacity demanded by today""s bandwidth requirements.
The present invention is a splice enclosure for use with fiber optic cables. The enclosure comprises a cannister including a tubular wall and an opening bounded by the tubular wall at a first end thereof The tubular wall, and consequently the opening bounded by the tubular wall, have an oblong-round shape which may be in the form of an elongated circle, ellipse, or oval. The opening is closed by an expansion seal assembly sized and shaped to match the opening and which includes a plurality of cable receiving ports for admitting the fiber optic cables into the splice enclosure.
The seal assembly comprises a pair of generally rigid plates and an elastomeric sealing layer sandwiched between the plates. The plates are connected by a plurality of threaded fasteners such that when the fasteners are tightened, the plates are drawn together, thereby causing the sealing layer to be compressed between the plates and expand radially until it conforms to the shape of the opening, sealing the enclosure.
The elastomeric sealing layer is divided into three portions along a pair of parting lines, the parting lines lying generally parallel to the long axis of the sealing layer. Each of the cable receiving ports includes a pair of aligned slots in the inner and outer plates, the slots each extending inward from the edge of the respective plate to a point proximate one of the parting lines. This arrangement allows an existing cable to be looped into the splice enclosure for repair without severing the entire cable.
The oblong-round shape of the enclosure provides several advantages over the cylindrical enclosures of the prior art: first, the interior capacity of the cannister is greater, allowing for more splices to be made within a single enclosure; second, the increased width allows for the use of wider splice trays, allowing more splices to be made in a single tray; third, the increased width allows for more buffer tube storage as required in mid-sheath splices; fourth, the increased width of the enclosure allows the cables to be spliced without risk of bending the cables at such a sharp radius that light transmission will be impeded; and fifth, the oblong-round seal assembly has more edge area than a comparably sized cylindrical splice, allowing more cable receiving ports to be provided and thereby allowing more individual cables to be spiced within a single enclosure.